Mention the latest discovery book

[Tomer-Eliahu]

Update the READMEs to mention the latest discovery book (https://docs.rust-embedded.org/discovery-mb2).

[BartMassey]

Thank you very much for this! I should have done something long ago: the plan is actually to archive this repo fairly soon.

That said, I think we might be a little stronger about the language. I think we should be really clear that people should be using the new book at this point unless they really need to work with a micro:bit v1 or an STM32F3 Discovery Board. If you want to do this that's great; otherwise let me know and I'll take your PR and add to it later.

Again, really appreciate this PR.

[Tomer-Eliahu]

I have made the changes to push people to use the new book.
Is this what you had in mind? If not, please let me know what other fixes are needed.

[BartMassey]

This looks great! Thanks very much.